This invention relates to insulating crystals which are coated on a metal mesh within a storage cathode ray picture tube in order to partially and/or totally control the amount of flood electrons passing through the metal mesh and reaching the phosphor screen, the control being effected by means of the persistent polarization and depolarization of the insulating crystals coated on the metal mesh.
In more detail, the present invention is related to a device which continuously and steadily displays instantaneous information on a phosphor screen for a long time period. The device includes a cathode ray picture tube which essentially contains: (a) a writing gun used to supply a sharply focused electron beam in order to write the instantaneous information on the insulating material coated on a metal mesh, said information being subsequently displayed on the phosphor screen; (b) flood guns which steadily supply an electron shower uniformly throughout on the insulating material on the metal mesh; (c) a metal mesh coated with an insulating material; (d) electrodes for collecting electrons repulsed from the insulating material on the metal mesh and for collecting secondary electrons which are emitted from the insulating material on the metal mesh and from the phosphor crystals; and (e) a phosphor screen which emits luminescence. In this tube, the incident electrons from the flood and/or writing guns, at first, pass through the metal mesh installed in front of the phosphor screen, and the incident electrons which are passed through the metal mesh penetrate into the phosphor crystals so as to make visible luminescence light from the phosphor screen. The pass of the flood electrons through the metal mesh, giving rise to the steady luminescence from the phosphor screen, is controlled by means of the properties of insulating crystals forming said insulating material and coated on the metal mesh utilizing the irradiation of a writing electron beam on the insulating crystals.
In a storage cathode-ray picture tube, the electrons from the flood guns (i.e. flood electrons) easily pass through the metal mesh, if there is no negative field on the metal mesh, and reach the phosphor screen, resulting in the luminescence thereof. Because the flood electrons are uniformly spread on the metal mesh (and therefore subsequently spread on the phosphor screen), the entire phosphor screen uniformly emits a steady luminescence, if there is no negative field on the metal mesh. However, the tube does not exhibit the storage of information if there is no negative field on the metal mesh.
In order to have the storage of information, it is absolutely necessary that the tube has a negative field over the entire area of the metal mesh (i.e. the ready mode), and the negative field is partially removed from those areas in which the writing electron beam has irradiated (i.e. the writing or storage mode). When the entire area of the metal mesh has a negative field, the flood electrons are repulsed by the negative field and are collected by the collecting electrodes; the flood electrons do not pass through the metal mesh, thereby resulting in no luminescence from the phosphor screen. When the partial areas of the metal mesh have no negative field, the flood electrons pass through those areas and those areas of the phosphor screen, corresponding to the areas on the metal mesh which have no negative field, emit the luminescence.
It has been believed that under irradiation of the flood electrons, the insulating crystals on the metal mesh are negatively charged due to the fact that the number of secondary electrons emitted from the crystals is less than the number of incident electrons entering the crystals; i.e. the ratio of the number of secondary electrons emitted to that of the incident electrons is less than one, e.g. .delta.&lt;1. Under the irradiation of the writing electron beam, the ratio is greater than one (e.g. .delta.&gt;1), resulting in positively charged crystals. The positively charged crystals on the metal mesh allow the flood electrons to pass through the metal mesh, and the passed flood electrons hit the phosphor crystal to produce luminescence. This concept leads to a conclusion that insulating crystals which have a high ratio of secondary electron emission may have an advantage when used as the crystals coated on the metal mesh in a storage tube. According to this conclusion, one is looking for insulating crystals which have a high ratio of secondary electron emission. However, there is no direct method to determine the ratio of secondary electron emission, and the ratio is traditionally estimated from the measurement results of the amount of the persistent charges on the surface of the insulators after irradiation by the electron beam. Using this technique, it has been determined that magnesium oxide (MgO) has the largest ratio. Thus, magnesium oxide has been coated on the metal mesh in storage tubes. However, the properties of magnesium oxide are very sensitive to the preparation conditions, and they are markedly changed from tube to tube. The reproducibility in the production of storage tubes is poor due to the poor reproducibility of the preparation of magnesium oxide, and a substitute insulator is desired.